The present invention relates to a beam structure for a web forming machine, which beam structure is arranged to be supported by its end components on the web forming machine.
Beam structures, which are generally supported on the web forming machine only by their end components, are used in different positions in a web forming machine, for example, in a paper or board machine. Usually the beam structure extends across the web forming machine for its entire width and is used to carry some device used in the process. Such devices are, for example, doctors, measuring devices, and coaters.
In the web forming process, temperatures are relatively high. In addition, the thermal load acting on the beam structure is often one-sided, which causes detrimental deflection in the beam structures due to the uneven thermal expansion. The deflection causes disturbances and errors in the operation of the device that is carried by the beam structure. For example, a doctor blade wears unevenly and the reading of a measuring device is erroneous. In known beam structures, a beam-like oscillation specific to them also appears, which is induced by the rotation of other devices, or generally by vibration arises from operation. The vibration further increases the disturbances caused by deflection. Particularly beam structures with a length of more than eight meters and which are used in connection with a web travelling at more than 1500 meters per minute, are large, expensive, and prone to vibration-related problems. In addition, in a web forming machine there are rolls, the rotational frequency, or semi-critical vibration of which will also induce vibration in a beam structure. This causes, for example, detrimental variations in the amount of coating in a coater.
In order to avoid deflection arising from thermal expansion, in known beam structures insulation is fitted around the load-bearing core structure. The insulation is intended to prevent heat being conducted to the core structure, so the temperature of the core structure will remain as even as possible. In order to protect the insulation, a casing structure is arranged, which also holds the insulation in place. However, the insulation with its casing structure also increases the total weight of the beam structure and thus also the deflection in the beam structure. This is because the insulation and the core structure are not load-bearing. Further, the insulation and its casing structure have no effect whatever on the vibration of the beam structure. At its worst, the casing structure itself can vibrate, which can induce vibration in the core structure.